Preterm Labor and Birth -- The Basics Explained

Reviewed by Traci C. Johnson, MD on July 20, 2021

Preterm labor can be scary. When a baby is born before they've had time to fully develop, it can have long- and short-term effects on their health.

But expecting parents shouldn’t worry too much -- only 12% of pregnancies in the U.S. result in early birth. And modern medicine has found ways to help many premature babies thrive.

A normal pregnancy lasts about 40 weeks. If you go into labor before 37 weeks, it’s called preterm labor, and your newborn is considered premature.

What Causes Preterm Labor?

Doctors often don't know exactly why it happens. And about half the time, there’s no known cause.

If you’ve had one preterm baby, you’re likely to have another. Other factors include:

  • High blood pressure in pregnancy (preeclampsia)
  • An ongoing (chronic) condition like kidney disease, heart disease, or diabetes
  • Carrying more than one baby
  • A problem with your uterus or cervix
  • Being younger than 17 or older than 35
  • An infection
  • Being seriously underweight at the time you got pregnant
  • Smoking
  • Using cocaine or other street drugs
  • Several miscarriages or abortions in the past
  • Not getting prenatal care from a qualified health care professional
  • A cervical infection
  • Jobs that require heavy, physical work
  • Stressful life events like the death of a loved one or domestic violence
  • Physical injury or trauma

How Does Early Birth Affect a Baby?

Not every preterm baby has health problems. But some that do need to stay in the hospital longer than those who go full term. They may spend time in the neonatal intensive care unit (NICU), where they can get round-the-clock care.

Breathing is a common problem for preemies, because their lungs aren’t fully developed. Your baby may need help from a ventilator, a machine that helps them breathe. They’ll need to be breathing on their own before you can take them home.

Your little one may also have trouble keeping a normal body temperature and will need to be kept warm. Doctors use incubators for this. Also, if they are born too early to suck and swallow, they’ll have to be fed through a needle in a vein or through a tube that passes through their nose and throat into their stomach.

It’s less common, but preemies sometimes can have other problems like:

  • Infections, such as pneumonia (lungs) and sepsis (a widespread infection in the blood)
  • Jaundice, which causes their eyes and skin to look yellow. It happens because their bodies can’t clear a substance called bilirubin that builds up in the liver.
  • Bleeding in the brain -- this sounds scary but it’s usually mild and resolves on its own
  • Low blood sugar, which can result from the low body temperature

As they get older, some of these children may have other troubles, ranging from difficulty with vision, to cerebral palsy, to learning struggles.

Show Sources


The Mayo Clinic. 

Sidelines National Support Network. 

Robin Elise Weiss, BirthCare Network.

March of Dimes: “Your Premature Baby.”

Mayo Clinic: “Premature Birth.” “A Primer on Preemies.”

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